I've not heard anything bad about the 3litre but others will reply later I'm sure. An automatic will never give you the same mpg as a manual and so far this has proved true with mine but in fairness it's swings n roundabouts.
If you end up buying a 2.7 you still won't be disappointed with it and reliability is fine too.
Big thing to look for is rust!
Sills, inside the rear arch there is a seatbelt mounting, boot floor and front inner wings/wheel arches. If the front wings are bubbling don't worry as thats normal and not an mot fail unless there are some sharp edges.
If you look at lwb then open rear passenger doors and lift the rear seats, there is a joint in the carpet. Lift the rear section of carpet at that point, check for rust as there is a body mounting in that location.
Unless it's badly rusted then most repairs aren't too bad.